It is not only considered very bad manners in Greece to make noise during the period from around 2 to 5 pm (and especially after 3 pm), but there is a law against it (though broken sometimes by those doing construction or using rototillers, because work is often excused if the workers have no other time to do that work). This is less so and rarely enforced in the city of Athens much to the chagrin of many lovsl residents. In theory it is quiet time. Siesta time!
So if a visitor to Greece is staying in a rooms complex that is near the houses of local Greeks, it is not appropriate to sit outside talking loudly and playing a radio or live music at that time.
The same need for quiet holds for those renting apartments in Greek towns or cities. During the hot summer months the midday meal and nap may get pushed forward until it cools off a little, with the meal even as late as 4 pm, and the nap to 5 pm or so. Few rules are 'set in stone' in Greece, but to be on the safe side, in the heat of summer, one should be aware that many Greeks are still napping until maybe 6:30.
Many visitors to Greece learn to do as the Greeks do, and find that this way of patterning one's day makes perfect sense, given the climate.